God has done marriage the honor of putting it into the Fourth Commandment, immediately after the honor due to Him, and com­mands, "Thou shalt honor father and mother." Show me an honor in heaven or on earth apart from the honor of God, that can equal this honor! Neither the secular nor the spiritual estate has been so highly honored. And if God had given utterance to nothing more than this Fourth Commandment with reference to married life, men ought to have learned quite well from this Commandment that in God's sight there is no higher office, estate, condition and work (next to the Gospel which concerns God Himself) than the estate of marriage.—"To the Knights of the Teutonic Order," Works of Martin Luther, Vol. III, pp. 423 f.

To get a wife is easy enough, but to love her with constancy is difficult, and he who can do that may well be grateful to our Lord God. Therefore if any one wants to marry a wife, let him take the matter seriously and pray to our Lord God: "0 Lord, if it is thy divine will that I should live without a wife, then help me to do so! If not, bestow upon me a good, pious maid, with whom I can live my whole life long, one whom I love and who loves me." For the mere union of the flesh is not sufficient. There must be con­geniality of tastes and character. — Conversations with Luther, p. 63.

When a man and a woman love and are pleased with each other, and thoroughly believe in their love, who teaches them how they are to behave, what they are to do, leave undone, say, not say, think? Confidence alone teaches them all this, and more. They make no difference in works: they do the great, the long, the much, as gladly as the small, the short, the little, and vice versa; and that too with joyful, peaceful, confident hearts, and each is a free companion of the other. But where there is a doubt, search is made for what is best; then a distinction of works is imagined whereby a man may win favor; and yet he goes about it with a heavy heart, and great disrelish; he is, as it were, taken captive, more than half in despair, and often makes a fool of himself.— " Treatise on Good Works," Works of Martin Luther, Vol. I, p. 191.

Divorce

Those who want to be Christians are not to be divorced, but each to retain his or her spouse, and bear and experience good and evil with the same, although he or she may be strange, peculiar and faulty; or, if there be a divorce, that the parties remain unmarried; and that it will not do to make a free thing out of marriage, as if it were in our power to do with it, changing and exchanging, as we please; but it is just as Jesus says: " What God has joined together let not man put asunder."

For trouble here is owing solely to the fact that men do not re­gard marriage according to God's word as his work and ordinance, do not pay regard to his will, that he has given to every one his spouse, to keep her, and to endure for his sake the discomforts that married life brings with it; they regard it as nothing else than a mere human, secular affair, with which God has nothing to do. Therefore one soon becomes tired of it, and if it does not go as we wish, we soon begin to separate and change. Then God nevertheless so orders it, that we thereby make it no better; as it then generally happens, if one wants to change and improve matters, and no one wants to carry his cross, but have everything perfectly convenient and without discomfort, that he gets an exchange in which he finds twice or ten times more discomfort, not alone in this matter but in all others. . . .

But you ask: Is there then no reason for which there may be separation and divorce between man and wife? Answer: Christ states here (Matt. v. 31-32) and in Matthew xix. 9, only this one, which is called adultery, and he quotes it from the law of Moses, which punishes adultery with death. Since now death alone dis­solves marriages and releases from the obligation, an adulterer is already divorced not by man but by God himself, and not only cut loose from his spouse, but from this life. For by adultery he has di­vorced himself from his wife, and has dissolved the marriage, which he has no right to do; and he has thereby made himself worthy of death, in such a way that he is already dead before God, although the judge does not take his life. Because now God here divorces, the other party is fully released, so that he or she is not bound to keep the spouse that has proved unfaithful, however much he or she may desire it.

For we do not order or forbid this divorcing, but we ask the gov­ernment to act in this matter, and we submit to what the secular authorities ordain in regard to it. Yet, our advice would be to such as claim to be Christians, that it would be much better to exhort and urge both parties to remain together, and that the innocent party should become reconciled to the guilty (if humbled and re­formed) and exercise forgiveness in Christian love; unless no im­provement could be hoped for, or the guilty person who had been pardoned and restored to favor persisted in abusing this kindness, and still continued in leading a public, loose life, and took it for granted that one must continue to spare and forgive him. . . .

In addition to this cause of divorce there is still another: if one of a married couple forsakes the other, as when through sheer petu­lance deserts the other. So, if a heathen woman were married to a Christian, or, as now sometimes happens, that one of the parties is evangelical and the other not (concerning which Paul speaks in I Cor. vii. 13), whether in such a case divorce would be right? There Paul concludes: If the one party is willing to remain, the other should not break the engagement; although they are not of one faith, the faith should not dissolve the marriage tie. But if it happens that the other party absolutely will not remain, then let him or her depart; and thou art not under any obligation to fol­low. But if a fellow deserts his wife without her knowledge or con­sent, forsakes house, home, wife and child, stays away two or three years, or as long as he pleases (as now often happens), and when he has run his riotous course and squandered his substance and wants to come home again and take his old place, that the other party must be under obligation to wait for him as long as he chooses, and then take up with him again: such a fellow ought not only to be forbidden house and home, but should be banished from the country, and the other party, if the renegade has been summoned and long enough waited for, should be heartily pro­nounced free. Commentary on Sermon on the Mount, pp. 169-174

…The third case for divorce is that in which one of the parties deprives and avoids the other, refusing to fulfill the conjugalFN1 duty or to live with the other person. For example, one finds many a stubborn wife like that who will not give in, and who cares not a whit whether her husband falls into the sin of unchastity ten times over. Here it is time for the husband to say, "If you will not, another will; the maid will come if the wife will not." Only first the husband should admonish and warn his wife two or three times, and let the situation be known to others so that her stubbornness becomes a matter of common knowledge and is rebuked before the congregation…FN2

Here you should be guided by the words of St. Paul, I Corinthians 7 [:4-5], "The husband does not rule over his own body, but the wife does; likewise the wife does not rule over her own body, but the husband does. Do not deprive each other, except by agreement," etc. Notice that St. Paul forbids either party to deprive the other, for by the marriage vow each submits his body to the other in conjugal duty. When one resists the other and refuses the conjugal duty she is robbing the other of the body she had bestowed upon him. This is really contrary to marriage, and dissolves the marriage…FN3

I therefore pass over the good or evil which experience offers, and confine myself to such good as Scripture and truth ascribe to marriage. It is no slight boon that in wedlock fornication and unchastity are checked and eliminated. This in itself is so great a good that it alone should be enough to induce men to marry forthwith, and for many reasons.

The first reason is that fornication destroys not only the soul but also body, property, honor, and family as well. For we see how a licentious and wicked life not only brings great disgrace but is also a spendthrift life, more costly than wedlock, and that illicit partners necessarily occasion greater suffering for one another than do married folk. Beyond that it consumes the body, corrupts flesh and blood, nature, and physical constitution. Through such a variety of evil consequences God takes a rigid position, as though he would actually drive people away from fornication and into marriage. However, few are thereby convinced or converted.

The estate of marriage, however, redounds to the benefit not alone of the body, property, honor, and soul of an individual, but also to the benefit of whole cities and countries, in that they remain exempt from the plagues imposed by God. We know only too well that the most terrible plagues have befallen lands and people because of fornication. This was the sin cited as the reason why the world was drowned in the Deluge, Genesis 6 [:1-13], and Sodom and Gomorrah were buried in flames, Genesis 19 [:1-24]. We see before our very eyes that God even now sends more new plagues.

In addition to these three grounds for divorce there is one more which would justify the sundering of husband and wife, but only in such a way that they must both refrain from remarrying or else become reconciled. This is the case where husband and wife cannot get along together for some reason other than the matter of the conjugal duty. St. Paul speaks of this in I Corinthians 7 [:10-11], "Not I but the Lord give charge to the married that the wife should not separate from her husband. But if she does, let her remain single, or else be reconciled to her husband. Likewise, the husband should not divorce his wife." Solomon complains much in the Proverbs about such wives, and says he has found a woman more bitter than death [Eccles. 7:26]. One may also find a rude, brutal, and unbearable husband.

Now if one of the parties were endowed with Christian fortitude and could endure the other's ill behavior, that would doubtless be a wonderfully blessed cross and a right way to heaven. For an evil spouse, in a manner of speaking, fulfils the devil's function and sweeps clean him who is able to recognize and bear it. If he cannot, however, let him divorce her before he does anything worse, and remain unmarried for the rest of his days. Should he try to say that the blame rests not upon him but upon his spouse, and therefore try to marry another, this will not do, for he is under obligation to endure evil, or to be released from his cross only by God, since the conjugal duty has not been denied him. Here the proverb applies, "He who wants a fire must endure the smoke."
What about a situation where one's wife is an invalid and has therefore become incapable of fulfilling the conjugal duty? May he not take another to wife? By no means. Let him serve the Lord in the person of the invalid and await His good pleasure. Consider that in this invalid God has provided your household with a healing balm by which you are to gain heaven. Blessed and twice blessed are you when you recognize such a gift of grace and therefore serve your invalid wife for God's sake.

But you may say: I am unable to remain continent. That is a lie. If you will earnestly serve your invalid wife, recognize that God has placed this burden upon you, and give thanks to him, then you may leave matters in his care. He will surely grant you grace, that you will not have to bear more than you are able. He is far too faithful to deprive you of your wife through illness without at the same time subduing your carnal desire, if you will but faithfully serve your invalid wife. Excerpts from 'The Estate of Marriage,' written in 1522 in 'Luther's Works,' Vol. 45, edited by Walter I. Brandt pg. 38-46

FN2 Mat. 18:15-18 But, if thy brother sin, withdraw, convince him, betwixt thee and him, alone,––If unto thee he hearken, thou hast gained thy brother; But, if he do not hearken, take with thee, yet one or two, that, at the mouth of two witnesses or three, every declaration, maybe established; But, if he hear them amiss, tell it to the assembly,––And, if, even the assembly, he hear amiss, Let him be unto thee, just as the man of the nations and the tax–collector.

FN3 1 Cor. 7:1-5 Now, concerning the things whereof ye wrote, it were, good, for a man, not to touch, a woman; But, on account of fornications, let, each man, have, his own wife, and, each woman, have, her own husband: Unto the wife, let the husband render what is her due, and, in like manner, the wife also, unto the husband,–– The wife, over her own body, hath not authority, but the husband, and, in like manner, the husband also, over his own body, hath not authority, but the wife. Be not depriving one another––unless perhaps by consent for a season, that ye may have leisure for prayer, and, again, may be together,––lest Satan be tempting you by reason of your want of self–control.